Density Effect of Eisenia sp. Epigeic Earthworms on the Hydraulic Conductivity of Sand Filters for Wastewater Treatment

Abstract

Inside sand filters, as inside other microporous substrates, several invertebrates create temporary burrows that impact on water movement through the filter. Lumbricids Eisenia fetida and Eisenia andrei live under a wide range of environmental conditions and have a high reproduction rate so they are good candidates for ecological engineering tests. We assessed the impact of these species at different densities (0, 100, 500, 1000 g m−2) on the hydraulic conductivity of small-sized experimental filters made of columns filled with filter sand classically used for sanitation mixed with 5% organic matter. The hydraulic conductivity was recorded every 7 days over 37 days in non-saturated conditions. On day 23, 40 g of peat bedding was added at the column surfaces to simulate a surface clogging organic matter pulse input. Columns with an earthworm density equal or superior to 500 g m−2 revealed the highest hydraulic conductivities during the first 21 days. At these densities, the hydraulic conductivity was also restored in less than 7 days after the addition of the surface organic matter, showing the influence of the earthworm species on the resilience capacity of the hydraulic conductivity. It was also highlighted that the hydraulic flow was dependent on the lumbricid densities with an optimal density/effect around 500 g m−2 in this specific substrate composition. This study showed that the feeding habits and burrowing activity of both Eisenia species significantly enhanced the hydraulic flow in a sandy substrate, providing a sustainable solution to limit the clogging of the substrate similar to the one used in filters to treat wastewater.